I use this system for my 2 sites by sharing the userbase though not for language purpose. No issues with it.
If it is just the language that is different - categories, tags, themes, plugins etc being same, I suggest the best way is to soft link to all Q2A directories from a different folder - especially qa-plugin and qa-themes which ensures site management will be easier.
I suppose a plugin can change the Q2A language per user- but this won't be working for the post contents. Again the post contents can be hidden per basis as per his language, but I do not see any advantage of this approach as contents being from different language means they should be in a different site.